Copper and zinc-containing superoxide dismutase (Cu,Zn-SOD), and manganese-containing superoxide dismutase (Mn-SOD) were purified from human placenta by heat treatment at 70 degrees for 5 min, following precipitation with ammonium sulfate and column chromatographies. The molecular weights of Cu,Zn-SOD and Mn-SOD estimated by gel filtration were 32000 and 81000, respectively. From the result of SDS polyacrylamide gel electrophoresis, Cu,Zn-SOD consisted of two subunits with an equal molecular weight of 16000 and Mn-SOD consisted of four subunits with an equal molecular weight of 21000. One g-atom each of copper and zinc was contained in the subunit of Cu,Zn-SOD, and one g-atom of manganese was contained in the subunit of Mn-SOD. Immunological properties of superoxide dismutases showed that no organ specificity existed but that a species specificity obviously existed. The method of Laurell's rocket immunoelectrophoresis which can determine Cu,Zn-SOD or Mn-SOD on the basis of the immunological difference has been established. The contents of Cu,Zn-SOD and Mn-SOD in various human tissues were measured by this method.